Question of the Day:Over the weekend I was in a discussion about hypothermia. We were discussing the "burrito wrap" and while wrapping the patient/victim in a Sleeping bag "burrito"; if a "thick" emergency blanket were available, should you place the em-blanket immediately next to the person or could/should it be placed on the outside of the sleeping bags as a sort of bivy?

The agreed upon theory that came up was the emergency blanket would act as a Vapor Blanket: but the "issue" was, being a VP, would the blanket accelerate or impede recovery if the VP was placed next to the skin?Part of the discussion was, the blanket as a VP, could/would make the body, as it was heating up, misread the retained vapor and inadvertently "turns down" the heat production (similar to what the body does in a normal situation when the skins feels warm moisture). Or would it speed up recovery if paced next to the skin?

If by "thick emergency blanket" you mean the scrimmed mylar emergency blanket, I would say that the best use in this situation would be to use it as a reflector to a fire built next to the victim. The lightweight mylar sheet works just as well for this as the "thick" one (which has just about 0 insulative value).

Steven, I think all parties would agree the perspiration might not be an issue in the moderate to severe cases; until the body starts warming up and then it could be an issue- that is what the question is all about. Can the VP at some point lessen the recovery?

Stephen, no not one of those cheesy thin Mylar sheets, it is a thick blanket like the thicknesses of 2 or more layers of a blue tarps with a Mylar coating on one side.Far more insulating value than the cheap Mylar sheets but nothing like a sleeping bag or regular blanket or even a thin fleece blanket.

I don't know whether putting the mylar radiant/vapor barrier outside or inside a sleeping bag would be best. I suspect the conditions would influence that decision. If the hypothermic person is wet, it might be better to put the mylar blanket inside the sleeping bag to protect the insulation from becoming damp. On the other hand, if the sleeping bag has a very light and breathable shell fabric and it's very windy, it might be better to put the mylar on the outside.

In any case, I think there is one important detail that is often overlooked when people use radiant barriers (like space blankets, etc.). The aluminized surface has not only a high reflectivity to IR wavelengths emitted by a person (in the neighborhood of 10 microns), but also a low emissivity. If the mylar goes inside the sleeping bag, near the skin, the aluminized side of the blanket should face OUT. In this case the blanket itself is relatively warm and it's job is to reduce IR emission. If it were turned around, aluminized surface facing in, the blanket itself would be just as warm and IR emission from the non-aluminized surface would be much greater. IR emissivity of most plastics (including mylar) is nearly 1. Putting a radiant barrier against your skin with the aluminum surface facing you reduces evaporative heat loss but not emission of radiant heat. It would be about the same as one layer of clear cellophane (the aluminum surface does nothing for you in this case).

If the mylar blanket goes outside the sleeping bag, the aluminized surface should face IN. In this case, in contrast to the first example, the mylar blanket is cold. Its job in this case is to reflect radiant heat emitted by the person. If it were turned around, aluminum side out, the blanket would still be cold, so very little would be achieved by the low emissivity of the outward-facing aluminum surface, and the inner surface, facing the person, would have a much higher IR absorbance.

These guidelines hold for use of radiant barriers in any situation, emergency or no. Radiant barriers that are aluminized on both sides are ideal because they work in almost any configuration.

I think that if you had the alumized layer on the outside, then you would have no radiant heat loss which is maybe 25% of your heat loss. http://en.wikipedia.org/wiki/Thermal_radiation

If you have aluminum layers inside, then you get some benefit but not as much - there is radiative heat transfer happening inside insulation

All the emergency blanket manufacturers have pictures with the shiny side facing in and the colored side facing out. Color at visible wavelengths is different than reflection at IR wavelength (emissitivity). If the colored side has less emisitivty, then it would be better facing in and shiny side facing out.

A lot of people talking about the shiny saide reflecting heat back to you, but I think it's more like you want the shiny side out so you don't emit so much IR.

As mentioned, the best way is to use another person's body heat for the rewarming phase. BUT given your question, I might say that using the e-blanket next to skin first with the sleeping bag over top might be faster initially. It can take a bit of time to warm-up a sleeping bag. Then, once the sleeping bag has warmed-up, I'd remove the e-blanket and put it on the outside to reduce the clammy VBL feeling.

The fastest way to warm someone up is to spoon inside a bag (or two), on a good pad, inside a tent. If you've got a third person, fire up the stove and start making hot water bottles. Hell, get a three-man spoon going...

Provided I had access to sleeping bags, jackets, shelters, etc., I wouldn't bother with space blankets at all.

If spooning a naked or half-naked partner in a real emergency is something someone wouldn't consider, then I don't think they should be a partner.

Spooning wasn't discussed in the original question, but is addressed below.

Colin and Jerry,What I hear you saying is that the aluminized surface should be facing out if next to the skin; almost like it is reflecting the colder air/wind outside away from the body. If not please explain.

Raymond,I even have trouble getting my wife to spoon when I'm warm and healthy; I guess I'm doomed if I every get in a bad situation (nobody will want to).

Spooning- I am of the opinion that "spooning" being the right thing to do for a Hypothermic person is a myth- developed by a male co-ed to entice an uninformed victim into a compromising situation.Spooning puts the rescuer into a situation where his/her health could suffer. It could cause the rescuer to become a liability, never something you want in a rescue situation.

or, if you have a fire, it's probably better behind you refecting radiation from fire

I think it would warm you up more to be in a bag with hypothermic person, because they would be warmer than ambient. In bag with hyperthermic person is no danger to you. You can wear a base layer and you don't have to cuddle to get benefit.

> opinion that "spooning" being the right thing to do for a Hypothermic person is a myth> Spooning puts the rescuer into a situation where his/her health could suffer.No worries, to each his own.

But you can be very sure that if it is a cold night, my wife and I sleep 'very close together'. And we can put one quilt over the other when we are that close. Benefits:* Heat loss from 2 sides rather than 3 sides* Body temperature 'hot water bottle' on the 3rd side* Double the amount of down insulation.

> entice an uninformed victim into a compromising situation.When it is -10 C outside???? You jest!

Back in 08 I recall hearing in Wilderness First Aid that spooning was no longer recommended. I believe there were two reasons give but my notes aren't handy so I could be wrong1. You don't want to create a second victim by getting another person in a sleeping bag with a cold and wet victim. 2. At some stages of hypothermia an extra body may not be that helpful.

If no one else comes up with more complete answers I'll try to get more info tomorrow but I gotta run soon.

I take a short (16 hour) "wilderness first aid" refresher course at least once every three years (BSA rules for backcountry outings) and also occasionally attend winter camping lectures by NOLS and Outward Bound staff. Not only has "spooning as hypothermia first aid" not been taught during the 21st century, it has been discouraged.

If you put the space blanket where it's warm (inside the insulation), the aluminum side needs to face out. If you put the space blanket where it's cold (outside the insulation), the aluminum side needs to face in. In the first case you are taking advantage of the low emissivity of the aluminum surface, and in the second case you are taking advantage of the high reflectivity of the aluminum surface.

I don't know what your heavy emergency blanket is made of. If it is one of the common nonwoven fabric + film heavy emergency blankets (and not fleece), and there is other insulation available (like a dry sleeping bag), you will almost certainly be able to trap a lot more heat with four standard mylar space blankets than with one heavy emergency blanket. If the victim doesn't have to be carried out (just warmed up), fragile space blankets might be a more effective tool.

but, if the aluminum side faces in, then it will touch the surface that's closer, and will conduct heat well because it's aluminum, so you're not taking advantage of the emissitivity/reflectivity

but, there is a small air gap, so maybe that's enough

is there a difference between reflectivity and absorbtion? I don't think so because the Stefan-Boltzman equation doesn't have different terms for refectivity and absorbtion.

I agree, flimsy (light) aluminized mylar is more weight effective - the actual layer of aluminum that does the reflecting weighs practically nothing - the substrate is where the weight is - which makes it hold up better to abuse, but in an emergency maybe the light weight aluminized mylar is sufficiently robust

Reflective barriers are more effective at reflecting heat, but rather ineffective at controlling heat loss.

Coming from the remodeling world, I am often asked about adding reflective materials up in an old attic to help with a houses r-value. The reality is that these materials are only effective when there is an unobstructed film of air between it and other surfaces. Even dust in an attic can degrade performance rather quickly.They can only help with reducing solar heat gain - not improving the warmth of a house in the winter.

Go to greenbuilding websites for more info on this: http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18717/reflective-insulation-attic

they say when installing radiant barrier to leave at least 1 inch air space between radiant barrier and hot inside of roof (they talk more about cooling than heating)

analog for space blanket - if the foil is facing inward, there's no air space at all so it wouldn't work so good. If the foil is facing outward on the outer layer, then there is an air space onext to the foil.

If the reflective side is facing outside, then the surface will only reflect any radiant energy that may aid in warming you up. In other words: the shiny surfaced blankets don't help, unless you toss it on the outside of your tent in order to reduce the heat gain if your tent in the summer.

Jerry, radiant barriers don't need an air space. They need a thermal insulator, and preferably an IR transmissive thermal insulator, at the IR reflective surface (the aluminum side). A lack of an IR transmissive thermal insulator is the reason that putting a space blanket against your skin, aluminum side in, does nothing to reduce radiant heat loss. Multilayer radiant barriers are very important to controlling the temperatures of satellite components, and space is a vacuum so there is no air space against those radiant barriers. Multilayer radiant barriers on spacecraft have fabric or scrim between the layers as a thermal insulator, to prevent heating of each layer by solid conduction from other layers ("thermal bridging"). An emergency blanket on the outside of a sleeping bag with the aluminum surface facing in would be in the same situation. It would have lightweight fabric against the reflective surface.

What would a space blanket on the outside of a sleeping bag be doing if the aluminum layer is facing out? If air temperature outside is 30F, what is the temperature of a space blanket on the outside of a sleeping bag? 33F? Freezing of moisture INSIDE the insulation of a sleeping bag is a problem for winter camping, and the outside of a sleeping bag is even colder. It wouldn't be doing a very good job as insulation if it was warm on the outside.

Any heat absorbed by a layer of cold plastic film exposed to cold air will be lost by convective cooling. The cold air will carry it away. The fraction lost to the environment by radiation will be vanishingly small, so you're not accomplishing anything by having the aluminized layer on the outside. But you are putting the mylar layer facing in, where it can absorb radiant energy from the deeper layers of the sleeping bag, which are warm. The space blanket then loses that energy to the outside air by convection.

If the aluminum side is facing in, the radiation from within the sleeping bag is reflected back toward the inner layers of the sleeping bag. Very little gets absorbed by the space blanket, so it can't be lost to the outside air.

"Jerry, radiant barriers don't need an air space. They need a thermal insulator, and preferably an IR transmissive thermal insulator"

All insulation for backpacking uses air. You can have a vacuum bottle or argon in a window but those aren't applicable to backpacking.

Anything like nylon or polyester will absorb IR if even a thin layer

An air space is the only thermal insulator relevant to backpacking that transmits IR

"Multilayer radiant barriers on spacecraft have fabric or scrim between the layers as a thermal insulator"

They have vacuum in between radiant layers, which has no thermal conduction. Any heat transfer is just at edges or wherever.

Because they are in vacuum it totally changes things. This is interesting, helps understand things, but isn't directly applicable.

In a house, you can suspend a radiant barrier and have an air space so that works.

A neo-air mattress does have suspended radiant barriers with air space between so that works.

In a sleeping bag, there's no way to suspend radiant barriers. If you had insulation in between to push out the radiant barrier, that insulation would absorb IR so it wouldn't work very good.

"What would a space blanket on the outside of a sleeping bag be doing if the aluminum layer is facing out? If air temperature outside is 30F, what is the temperature of a space blanket on the outside of a sleeping bag? 33F?"

If you're under a clear sky, then the surface of a regular sleeping bag will emit heat so it will be at the same temperature as ambient. If it was aluminum covered and you didn't have that heat loss, it would be 10 degrees warmer than ambient.

That computation doesn't provide any information at all about the temperature of the outside of a sleeping bag. It would be very elegant if it did.

Our disagreement on a few points has made this a thought-provoking discussion, but I assume we can agree that cooling of the surface of a sleeping bag is dominated by convection, not radiation. Any heat not lost from the surface as radiation, due to low emissivity, will be lost to the cold air (convective cooling rates increase with increasing temperature gradients even when the bulk movement of air is unchanging). The best way to reduce heat loss from the surface, in my opinion, is to reduce the amount of heat that gets there in the first place by facing the reflective surface inward. I grant that the solid materials of the insulating fibers in a sleeping bag (keratin or polyester) are not very IR transmissive, but the air between them is.

The Stefan-Boltzman equation tells how much power is emitted from a surface

But I wonder how that translates to temperature difference. For example, will the outer surface temperature go below ambient?

I don't think there are any products that use an aluminum surface except emergency blankets, so our discussion is just academic.

If I was in an emergency, I'de put the blanket on the outside, to keep out rain, and wind, and prevent warm air from drifting away (convection), and I think it would also minimize radiative heat loss : )

Someone should make a fabric for the outside of a sleeping bag, with an aluminized surface, and holes for water vapor to exit through. Like 5% or 10% of the surface would be holes, then it would be 90% or 95% effective at not radiating.

"In a sleeping bag, there's no way to suspend radiant barriers. If you had insulation in between to push out the radiant barrier, that insulation would absorb IR so it wouldn't work very good."

And that perfectly describes the whole paradox. For the barrier to be at all helpful in this situation, Two things must take place:1) It would need to face inward, however:2)It could not touch the skin of the person whatsoever, or have any obstructing material between it and the skin, in order to adequately reflect energy back.

The second the shiny material touches the surface of the skin, it would no longer act as an emitter, and become more of a conductor. The R-value this product is negligible, so it would not aid in insulating. Thus, any material between it and the skin would not only severely cut down on the radiant heat energy being lost by the person, but would cut out any reflected energy from entering the system.

In my opinion, emergency blankets:1) Could only be marginally useful when an actual blanket is not around.2) Should not even be marketed as a "blanket", since a tent ground cover likely has a higher R-value than them.

On a semi-related note (and to illustrate the never ending confusion between emissivity and insulation:) here is the unfortunate story of a person who was so convinced of the "thermal qualities of insulating reflective paint", that he forced his builder to use the paint instead of insulation. He sued the HVAC Contractor thinking his homes heating and cooling problems were his fault.

Kelty used to sell sleeping bags in the 80's with a shiny interior surface. I don't think it lasted too long. Regardless of the lack of products in the outdoor world, there is much data on these types of materials in the building world:

There are pretty good reasons why you DON'T see any building codes require it in their wall/envelope assemblies. These products have only been reasonably helpful in reducing solar heat gain, but not keeping heat from leaving us. But regardless of their performance, they can't come close to the performance of simply having more insulation.

They say it has 8 togs of thermal insulation value = 5 clo which would be good down to maybe 20 degrees F. That is very good compared to a regular sleeping bag.

Except tog/clo/R/m2K/W all are conduction measurements. Radiation doesn't directly convert to that unit. However, you could measure temperature gain with it and use the conduction unit of measure to produce the same temperature difference. It would depend on whether the sky was clear or cloudy or you were in a sheltered location.

They measured at Leed's University which has the best reputation

There was one diagram that showed 3 layers of aluminized material with air in between cells

The only problem is it doesn't pass water vapor, but you could use VBL

Once again, they had a picture with the colored side out and the shiny side facing in

And you can get it in 3 colors - what's that about? - marketing???

But, they said that if you have two space blankets on top of each other that the 2nd blanket adds little extra warmth. That tells me that to be effective, the aluminized layer either has to be on the outside, or facing an air space. If you face the aluminized layer in, then it will be against whatever clothing you're wearing - maybe a big enough air space on average to be a little effective.